Work has long been ongoing on producing adhesive tapes without using solvents or at least on designing the coating operation and the downstream steps to be solvent-free. Corresponding products with films of adhesive based on synthetic rubbers are known. These products, however, cover only the lower-end performance range of adhesive tapes.
For a number of years it has also been possible to obtain solvent-free adhesives based on acrylate, which can be processed further as hotmelt adhesives for adhesive tapes. Normally, however, they do not match the shear strengths of acrylate compositions applied from solution. One important reason for this is that the viscosity of the compositions during processing must not become too high, since otherwise the operations of melting and of coating onto a carrier are too expensive from the economic standpoint. The viscosity is determined substantially by the length of the molecules. Relatively short chain molecules, however, result in poorer shear strengths. Even crosslinking of the adhesive following its application permits only limited improvement.
In the case of natural rubber adhesive systems, the melting operation can be avoided if success is achieved in mixing the components of the composition without solvent and if the hot composition is supplied directly to a coating system. Examples of suitable mixing units include extruders. In the course of mixing, however, the rubber must not be degraded any more than slightly, since otherwise the product properties are impaired.
In the case of acrylate systems, the melting operation can be avoided by removing the solvent or water from compositions polymerized in solvents or in water, the removal taking place inline for the purpose of coating. In an appropriate extruder, for example, solvents or water can be removed by way of vacuum zones.
For the application of high-viscosity compositions, slot dies are suitable. It is found that they are also suitable for high-viscosity adhering compositions as described above. However, above a relatively low web speed, air bubbles become included between the adhesive and the substrate, which is typically coated on a coating roller.
In order to reduce the formation of bubbles in the above task, the market recommends blowing dies, suction dies, and what are known as vacuum boxes. The aim of using these devices is to raise the force with which the composition is pressed against the substrate.
Known from film manufacture (for example, EP 0 920 973 A2) are wire, blade, and needle electrodes, disposed transverse to the web, with which electrical charges are applied to the composition that is to be laid on. By this means the composition is pressed by electrostatic forces against a metal roller. Furthermore, combinations of electrostatic forces and forces by air movement are also used (EP 0 707 940 A2).
For the above-described coating of substrates, the literature reports complex solutions, in which before being laid onto the chill roll the substrate is charged in multistage upstream processes, partially discharged by heating, and cooled, in order finally to obtain uniform charging of the substrate on the coating roller (for example, EP 0 299 492 A2).
The maximum level of charge on the substrate in these cases, however, is relatively low, since as early as on leaving the charging roller it is reduced until the air is no longer ionized, owing to the electrical field strength resulting from the charge density.
From film manufacture (see, for example, U.S. Pat. No. 4,997,600 A1) a roller insulation system is known in which prior to the laying-on of the film electrical charges are applied to the insulator layer in order to increase the forces of applied pressure when the film is laid onto the roller.
If no charges are applied to the insulated roller, the electrostatic forces applying pressure during the laying-on operation are greatly attenuated as the thickness of the insulator layer goes up. At the required insulator thicknesses needed for sufficient high-voltage resistance for the ceramic coatings specified here, achievable bubble-free coating speeds are drastically reduced.
DE 199 05 935 Al discloses a method of producing a coating of solvent-free pressure sensitive adhesive systems on substrates, especially release-coated substrates, in which                the pressure sensitive adhesive system is applied in one or more layers to a rotating roller by means of an adhesive applicator,        the pressure sensitive adhesive system on the roller is crosslinked in an exposure means by high-energy radiation, specifically by means of electron beams (EB), UV or IR radiation, and        the roller is contacted with the substrate, so that the pressure sensitive adhesive system is transferred from the roller to the substrate, and where appropriate is rolled up.        
Typical exposure means employed in the context of the embodiment of the method that is depicted in said patent include linear cathode systems, scanner systems, and multiple longitudinal cathode systems, where electron beam accelerators are concerned. The acceleration voltages are situated in the range between 40 kV and 350 kV, preferably from 80 kV to 300 kV. The output doses range between 5 and 150 kGy, in particular from 20 to 90 kGy.
As UV crosslinking units it is possible in particular to employ two medium pressure mercury lamps each with an output of 120 W/cm or one medium pressure mercury lamp having an output of 240 W/cm. The doses set are preferably from 10 to 300 mJ/cm2.
DE 199 05 935 A1 describes a method of producing a coating of solvent-free pressure sensitive adhesive systems on substrates, especially release-coated substrates, in which                a fluid film is applied to a rotating roller by means of a fluid applicator,        the pressure sensitive adhesive system is applied in one or more layers to the fluid film by means of an adhesive applicator, so that the fluid film is located between roller and pressure sensitive adhesive system, and        the roller is contacted with the substrate, so that the pressure sensitive adhesive system is transferred from the roller to the substrate (release-coated or otherwise).        
The contacting of the substrate takes place in particular by way of a second roller. Substrates used include papers, films, nonwovens, and release-coated materials such as release papers, films, and the like.
The second roller, also referred to as a contact roller, may have been provided with a rubber coating and is pressed against the roller with a linear pressure of preferably 50 to 500 N/mm, in particular from 100 to 200 N/mm. The contact roller preferably has a Shore hardness (A) of from 40 to 100, in particular a Shore hardness of from 60 to 80 shore (A). The substrate is preferably brought into contact with the roller in such a way that the speed of the roller surface coincides with that of the substrate. Where, however, it is intended that a reduction in thickness should take place along with the removal of the adhesive film, the substrate may also have a higher speed.
In one advantageous embodiment the roller is a steel roller, a chrome-plated steel roller, a rubber roller or a silicone rubber roller and/or is manufactured from elastic material. Furthermore, the roller may be smooth or may have a slightly structured surface. The smooth roller may preferably have a chrome coating. Optionally, the chrome-coated steel roller may possess a high-gloss-polished surface with a roughness Rz<0.02 μm. The coating roller may also, however, be rubberized, preferably with a rubber hardness of from 40 to 100 shore (A), in particular with a hardness of from 60 to 80 shore (A). The roller coating may, in accordance with the prior art, comprise EPDM, Viton or silicone rubber, or other elastic materials.
It has also proven advantageous for the roller to be temperature-controllable, preferably in a range from −10° C. to 200° C., particularly from 2° C. to 50° C.
It is an object of the invention to allow the substrate to be coated with compositions, especially compositions of high viscosity, such as are used for producing adhesive tapes or similar products, with the preferred use of a slot die, with sufficiently high web speeds. In the course of this operation, there should be                no bubbles included between the composition coat and the substrate,        no detriment to quality-critical properties of the product being produced, and        no hazards arising for operating staff.        
This object is achieved by means of a process as specified in the main claim. The subclaims describe advantageous embodiments of the process.